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EE2301 POWER ELECTRONICS
UNIT I
POWER SEMICONDUCTOR DEVICES
Power Diode
1. What is fast recovery diode?
(N/D 04)
The fast recovery diode has low recovery time, normally less than 5 microsec. They
are used in choppers, SMPS, induction heating
2. What is reverse recovery time?
(A/M 06)
The reverse recovery time is defined as the time between the instant, forward diode
current becomes zero and 25 % of the maximum reverse current.
Power BJT
3. What are the advantages of GTO over BJT?
1.
2.
3.
4.
(N/D 04)
Higher voltage blocking capability
It is majority carrier Devices
High ratio of peak surge current to average current
Operates as a switch always
4. What are the advantages of MOSFET over BJT?
MOSFET
Power MOSFET has lower switching
losses
(A/M 08)
BJT
BJT has higher switching losses
It has low conduction losses
It has more conduction losses
It is current controlled device
It is a voltage controlled device
It is a bipolar device
It is a unipolar device
5. Why is a BJT called a current controlled device?
Power BJT is a current controlled device, because the output (collector) current can be
controlled by base current.
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6. Draw the V–I characteristics of SCR and mark the holding current and the latching
current on the characteristics.
(N/D 04)
7. Define – Holding Current and Latching Current of SCR
(A/M 08)
The holding current is defined as the minimum value of anode current below which it
must fall to for turning off the thyristor.
The latching current is defined as the minimum value of anode current which it must
attain during turn on process to maintain conduction when gate signal is removed.
8. Write any two advantages of GTO over SCR.
1. GTO has faster switching speed
2. It has more di/dt rating at turn on
9. What are the factors that influence the turn-off time of a thyristor?
Some factors that influence the turn off time of a SCR are,
1. Recovery process
2. Recombination process
(A/M 04)
(N/D 06)
10. What is meant by the turn off time of a converter grade SCR? Mention its value.
SCR with slow turn off time is called converter grade SCR. The turn off time for
converter grade SCR is 50-100 ms.
11. What is meant by the turn off time of an inverter grade SCR?
SCR with fast turn off time is called inverter grade SCR. The turn off time for inverter
grade SCR is 3-50 ms.
12. What are the different methods of turning on of a thyristor?
The following methods are used to turn on the thyristor:
1. Forward voltage triggering
2. Gate triggering
3. dv/dt triggering
4. Temperature triggering
5. Light triggering
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Power MOSFET
13. Why are MOSFETs not preferred for low frequency applications?
(M/J 06)
MOSFETs have high switching losses so it can’t use low frequency application.
14. Define – Pinch Off Voltage of MOSFET
(N/D 07)
If the gate source voltage VGS is made negative enough, the channel will be
completely depleted, offering a high value of RDS and there will be no current flow from the
drain to source, IDS=0. The value of VGS is called pinch off voltage.
Power IGBT
15. Why are IGBTs becoming popular in their applications in controlled converters?
IGBT is very popular nowadays because it has,
1. Lower gate requirements
2. Lower switching losses
3. Smaller snubber circuit requirements
TRIAC
16. What are the advantages of TRIAC?
The advantages of TRIACs are,
1. TRIACs can be triggered with positive or negative polarity voltage.
2. A TRIAC needs a single fuse for protection, which also simplifies the construction
3. A TRIAC needs a single heat sink of slightly larger size, whereas anti-parallel
thyristor pair needs two heat sinks.
4. In some DC applications, SCR is required to be connected to be connected with a
parallel diode to protect against reverse voltage, where a TRIAC used may work
without diode as safe breakdown in either direction is possible.
UNIT II
PHASE CONTROLLED CONVERTERS
Two pulse converter
1. What is a two pulse converter?
(N/D 11)
Two pulse converter is defined as two triggering pulses or two sets of triggering
pulses are to be generated during every cycle of the supply to trigger the various SCRs.
2. What is meant by full converter?
(N/D 04)
A fully controlled converter or full converter uses thyristor only and there is a winder
control over the level of DC output voltage. It is also known as two quadrant converter.
3. Define – Firing Angle α
(A/M 04)
The firing angle is defined as the angle between the zero crossing of the input voltage
and the instant the thyristor is fired.
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4. List out the merits of phase control of SCR.
Merits of phase controlled SCR are
1. Controlled output voltage
2. Less harmonics
(A/M 04)
5. Why is the power factor of a semiconverter better than that of a full converter?
(A/M 08)
When supply is given to load, the semiconverter receives less reactive power due to
freewheeling action when compared with full converter. Therefore, the power factor is better
in semiconverter.
6. What are the conditions under which a single phase fully controlled converter
operates as an inverter.
(N/D 07)
In line commutated converter, when the firing angle is 90 º to 180 º it acts as a
inverter.
7. List out the applications of phase controlled converters.
(N/D 06)
The applications of controlled rectifiers are,
1. Steel rolling mills, printing press, textile mills and paper mills employing DC motor
drives.
2. DC traction
3. Electro chemical and electro-metallurgical process
4. Portable hand tool drives
5. Magnet power supplies
6. HVDC transmission system
8. What is a half controlled rectifier?
A half controlled rectifier or semiconverter uses a mixture of diodes and thyristors
and there is a limited control over the level of DC output voltage. It is also known as onequadrant converter. Here, the output current and output voltage is always positive.
9. What is the function of a free-wheeling diode in controlled rectifier circuits?
(N/D 06)
1. It serves two processes.
2. It prevents the output voltage from becoming negative.
3. The load current is transferred from the main thyristors to the freewheeling diode,
thereby allowing all of its thyristors to regain their blocking states.
10. What is meant by commutation?
It is the process of changing the direction of current flow in a particular path of the
circuit. This process is used in thyristors for turning it off.
11. What is meant by forced commutation?
In this commutation, the current flowing through the thyristor is forced to become
zero by external circuitry.
12. What is meant by natural commutation?
Here the current flowing through the thyristor goes through a natural zero
the thyristor to turn off.
and enable
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13. What are the different types of commutations?
The different types of commutations are
1. Natural commutation
2. Forced commutation
14. What is inversion mode in a single phase fully controlled converter?
In a single phase full converter, α>90°, the voltage at the DC terminals is negative.
Therefore, power flows from load to source and the converter operates as a line commutated
inverter as source voltage VS is negative and source current is positive. This is known as
inversion mode or synchronous inversion.
15. Define – Commutation Angle or Overlap Angle
The commutation period when outgoing and incoming thyristors are conducting is
known as overlap period. The angular period, when both devices share conduction is known as
the commutation angle or overlap angle.
Performance characteristics
16. Define – Displacement Factor and Total Harmonic Distortion
(M/J 07)
The input displacement factor is defined as the cosine of the input displacement angle.
The harmonic factor of the input current is defined as the ratio of the total harmonic current
to the fundamental component.
IH = ( I2rms – I21) / I1
Effect of source inductance
17. What are the effects of source impedance in a controlled rectifier?
The effects of source impedance in the controlled rectifiers are,
1. The average output voltage is reduced
2. Reduced displacement factor
3. Output current waveform is changed
(N/D 06)
UNIT- III
DC TO DC CONVERTERS
Chopper
1. Define – Duty Cycle of DC Chopper
(N/D 06, 07)
Duty cycle means the ratio of the on time of the chopper to the total time period of the
chopper. It is denoted by α.
2. What is a DC Chopper?
(N/D 07)
A DC chopper is a high speed static switch used to obtain variable DC voltage from a
constant DC voltage.
3. What are the applications of DC Choppers?
The applications of DC choppers are,
1. Battery operated vehicles
2. Traction motor control in electric traction
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3.
4.
5.
6.
Trolley cars
Marine hoists
Mine haulers
Electric braking
Time ratio and current limit control
4. What are the control strategies used in DC choppers?
The types of control strategies in DC choppers are,
1. Time Ratio Control (TRC)
2. Current Limit Control method (CLC)
(N/D 03)
5. What are the disadvantages of FM scheme used in Chopper?
1. Filter design is very difficult
2. FM scheme produces interference
(N/D 04)
6. What are the methods of controlling the output voltage of a chopper?
The methods of controlling the output voltage of a chopper are
1. Time Ratio Control (TRC)
2. Current Limit Control method (CLC)
(A/M 05)
7. Differentiate constant frequency control strategy from variable frequency control
strategy of varying the duty cycle of DC choppers.
Constant frequency control
Chopping frequency is constant
It has fast response
Variable frequency control
Chopping frequency is varied
It has slow response
8. What is meant by time ratio control of DC–DC converter?
In time ratio control (TRC), the value of Ton / T is varied in order to change the
average output voltage.
9. What is meant by pulse width modulation control in a DC chopper?
(M/J 07)
The PWM control method in DC chopper means, the on time T on is varied but
chopping frequency ‘f’ is kept constant. The width of the pulse is varied and this type of
control is known as Pulse Width Modulation (PWM).
Buck regulator
10. What are the different types of switching regulators used in DC Choppers?
The different switched mode regulators available are,
1. Buck converter
2. Boost converter
3. Buck – Boost converter
4. Cuk converter
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11. What is meant by buck regulator?
(M/J 07)
The buck converter is defined as, a converter used to step down an unregulated DC
input voltage to regulated variable DC output voltage.
Chopper classification
12. What are the classifications of DC to DC converter depending upon the directions of
current and voltage?
(M/J 06)
Depending on the direction of current and voltage, choppers can be classified into the
following types,
1. Type A chopper or first quadrant chopper
2. Type B chopper or second quadrant chopper
3. Type C chopper or two quadrant type A chopper
4. Type D chopper or second quadrant type B chopper
5. Type E chopper or four quadrant chopper
13. What is two-quadrant DC chopper?
(A/M 08)
A DC chopper can be operated in the I and II quadrant as well as I and IV quadrant.
Here the output voltage is always positive but output current is either positive or negative.
SMPS
14. What are the applications of SMPS?
The applications of SMPS are,
1. Computer
2. Television receiver
3. Battery charger
UNIT IV
INVERTERS
Single phase inverter
1. List out the industrial applications of inverters.
The applications of inverters are,
1. Adjustable speed drives
2. Induction heating
3. Stand-by aircraft power supplies
4. UPS
5. HVDC transmission
(A/M 04)
2. What is the purpose of connecting diode in antiparallel with thyristors, in inverter?
(A/M 04)
For RL loads, load current will not be in phase with load voltage and the diodes
connected in antiparallel will allow the current to flow when the main thyristors are turned
off. These diodes are called feedback diodes.
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3. What is meant by inverter?
An inverter means a device that converts DC power into AC power at desired output
voltage and frequency.
PWM technique
4. What is PWM?
(A/M 05)
PWM control means, a fixed DC input voltage is given to the inverter and a controlled
AC output voltage is obtained by adjusting the on and off periods of the inverter components.
This is the most popular method of controlling the output voltage and this method is termed
as PWM control.
5. Define – Modulation Index of PWM.
(N/D 05)
The amplitude modulation index defined as the ratio of reference voltage ER to the
control voltage EC and it is denoted by M.
M = ER / Ec
where,
M - Modulation index
ER - reference voltage
Ec - control voltage
6. What are the modulation techniques used in an inverter?
1. Single pulse width modulation
2. Multi pulse with modulation
3. Sinusoidal pulse width modulation
(M/J 06)
7. What are the advantages of PWM control?
The advantages of PWM control are,
1. The output voltage can be obtained without any additional components
2. Lower order harmonics can be eliminated or minimized along with its output voltage
control. As the higher order harmonics can be filtered easily, the filtering
requirements are minimized.
Current source inverter
8. List out the applications of CSI.
The applications of a CSI are,
1. Induction heating
2. Lagging VAR compensation
3. Speed control of AC motors
4. Synchronous motor starting
9. Differentiate VSI from CSI.
Sl. No.
1
VSI
Input voltage is maintained
constantly
The output voltage does not
(N/D 04)
(N/D 03, 06, 07)
CSI
Input current is constant but
adjustable
The output current does not depend
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2
3
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depend on the load
The magnitude of the output
current and its waveform
depends on the nature of the
load impedance
It requires feedback diodes
Commutation circuit is
complicated i.e. it contains
capacitors and inductors.
on the load
The magnitude of the output voltage
and its waveform depends on the
nature of the load impedance
It does not require feedback diodes
Commutation circuit is simple i.e. it
contains only capacitors.
10. What are the advantages of current source inverter?
The two advantages of CSI are,
1. CSI does not require any feedback diodes.
2. Commutation circuit is simple as it involves only thyristors.
(M/J 07)
UNIT V
AC TO AC CONVERTERS
AC voltage controller
1. What is an AC voltage controller?
(M/J 10)
An AC voltage controller means it converts fixed alternating voltage into a variable
voltage without change in frequency.
2. What are the applications of AC voltage controllers?
The applications of AC voltage controllers are,
1. Domestic and industrial heating
2. Lighting control
3. Speed control of single phase and three phase ac motors
4. Transformer tap changing
3. What are the types AC voltage controllers?
1. Single phase AC voltage controller
2. Three phase AC voltage controller
(M/J 13)
4. What is meant by unidirectional or half-wave AC voltage controller?
(N/D 11)
In a unidirectional or half-wave AC voltage controller, the power flow is controlled
only during the positive half-cycle of the input voltage.
5. What is meant by bidirectional or half-wave AC voltage controller?
(N/D 10)
In a bidirectional or half-wave AC voltage controller, the power flow is controlled
during both cycles of the input voltage.
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Cycloconverter
6. What is meant by cycloconverter?
(N/D 12)
A cycloconverter converts input power at one frequency to output power at another
frequency with one-stage conversion. Cycloconverter is also known as frequency changer.
7. What are the two types of cycloconverters?
The two types of cyclo-converters are,
1. Step-up cyclo-converters
2. Step-down cyclo-converters
8. What are the applications of cycloconverters?
The applications of cyclo-converters are,
1. Induction heating
2. Speed control of high power ac drives
3. Static VAR generation
9. What is meant by negative converter group in a cycloconverter?
The negative converter group in a cycloconverter is the part of the cycloconverter
circuit that permits the flow of current during the negative half cycle of output current is
called negative converter group.
10. What is a positive converter group in a cycloconverter?
The positive converter group in a cycloconverter is the part of the cycloconverter
circuit that permits the flow of current during the positive half cycle of output current is
called positive converter group.